High load mercury switch



June 11, 1946. c. H. LARSON HIGH LOAD MERCURY SWITCH Filed Oct 16, 1945 IN VEN TOR. [J17 garl JY lax-.5011

Atty;

\ Patented June 11, 1946 HIGH LOAD MERCURY SWITCH Carl H. Larson, Elkhart, Ind., assignor to The Adlake Company, a corporation of Illinois Application October 16, 1943,'Serial No. 506,493

8 Claims.

The principal object of this invention is to increase the life and current carrying capacity of mercury switches, particularly those of the metal envelope, mercury displacement type. Heretofore, a switch of this type which could carry 20 amperes at 110 volts D. G. with an inductive load was considered a heavy duty switch, and its performance about the best that could be achieved in such a switch. With the present invention, however, the current carrying capacity of switches of this type has been at least tripled without shortening the life of the switch.

One problem involved in the operation of a high load mercury switch arises from the fact that mercury has both unfavorably high resistivity or specific resistance and unfavorably low specific heat. On the one hand, the effectiveness of a given electric current to generate heat by internal resistance is more than 55 times as great in mercury as in copper and, on the other hand, such internally generated heat has a temperature-raising effect on mercury three times as great as on copper.

Another problem resides in the necessity first, of

employing metal for a high pressure switch envelope and second, of using a metal displacement member in the envelope to serve as an armature for electromagnetic actuation of the switch. Since one terminal of such a mercury switch is connected to the metal envelope and since the resistivity of the armature is lower than the re sistivity of the mercury, current in the switch circuit tends to flow through the displacement member to or from the envelope. This tendency may be readily understood when it is considered that the resistivity of iron is approximately one eighth the resistivity of mercury,

The tendency for current to flow between the metal envelope and the enclosed armature increases with the switch current and is highly accentuated if the armature is in close proximity to the wall of the envelope. It is desirable to place the material of the armature close to the envelope wall both for the purpose of keeping the armature spaced away from the central arcing zone of the switch and for the purpose of causing the armature to be guided in. its operative movement by the surrounding envelope wall.

In. the type of displacement mercury switch in which the present invention has special utility, a cylindrical armature is surrounded. by a cylindrical metal. wall of the envelope and the armature is usually provided with peripheral guiding portions or radial spacer projections for cooperation with the surrounding wall, The coacting metal surfaces are smooth for minimum friction,

.amperes or more, will tend to melt the two metals and in a relatively short period of service result in such roughened and pitted coacting surfaces as to seriously interfere with freedom of movement of the armature. Sustained current flow substantially above '20 amperes may, in a conventional switch construction, actually weld the armature and envelopetogether.

A feature of my invention is the concept that the two problems of avoiding excessive mercury temperature and of avoiding serious deterioration of the coacting surfaces maybe solved. by the single expedient of properly extending a conductor of low resistivity and high current capacity from the metal envelope to the make-and-break zone of the switch. This expedient efficiently accomplishes its purpose, first, because it shortens the path of current flow through mercury and, second, because it affords a path of current from the envelope to the make-and-break zone of so much higher conductance than any possible path through the armature that no significant or damaging amount of current can flow between the envelope and the armature.

It is to be noted that shortening of what may be termed the mercury path by employment of the low resistance conductor enters into the solution of both problems since shortening the distance of current flow through the mercury both reduces the heating action of the current on the mercury and favors current flow through the conductor rather than through the armature. In my preferred construction, the conductance differential in favor of the conductor is increased by making the conductor relatively large in cross section and may be further increased by fabricating the conductor from material having substantially lower resistivity than the material of the armature.

One of the special objects in the preferred practice of my invention relates to the construction and mounting of a nonmetallic cup to hold a pool of mercury around one of the switch electrodes. In certain prior art practices, the ceramic material of a cup wall is fused or otherwise bonded to some other material either for the purpose of building up the cup itself or the purpose of mounting the cup on a suitable support. It has been found that any necessity for such bonding limits the choice of cup materials. If

fusing is employed, the most desirable ceramics may be precluded by the necessity of matching the interjoined materials with respect to their coeflicients of thermal expansion. It has also been found that any necesslty to employ a bond ing agent involves certain difiiculties since many bonding substances deteriorate in the presence of mercury, often with the resultant production of undesirable impurities.

The problem of mounting a nonmetallic cup on an electrode or other support is especially troublesome in those prior art arrangements in which some mounting means extends through and is bonded in an aperture in the bottom of the cup. Any deterioration of a bond in such location provides a short-circuit path of mercury through the cup aperture to make the switch inoperative or at least operative only in a faulty manner. With respect to the fabrication and mounting of a cup to contain a mercury pool, it is my object to provide a simple and effective mode of mounting a nonmetallic cup in the form of a unitary and imperforate body of material whereby bonding substances may be dispensed with entirel or only simple bonding substances or agents may be required. Since the cup body is imperforate, no bonding material is required to function as a mercury seal to prevent short circuiting and therefore any bonding substances or expedients employed need function merely as a mechanical con.- necting means with no consideration for leakage.

Other objects of my invention relate to meth ods and materials pertinent to the cost of manufacture and the ease of assembly of mercury switches. Some of these objects are to eliminate glass blowing thereby both to lower the cost of manufacture and to avoid excessive variation in the dimensions of fabricated parts; to provide for increased use of automatic machinery in the fabrication of mercury switches; to decrease the number and extent of testing operations required in the course of manufacture; and to achieve such uniformit in the switch parts as to simplify assembly and make possible the manufacture of substantiall uniform subassemblies from randomly selected parts.

The above and other advantages of my invention will be apparent in the detailed description to follow taken with the accompanying drawing.

In the drawing. which is to be regarded as illustrative only:

Fig. 1 is a sectional view of a preferred form of my switch in one of its two operative positions:

Fig. 2 is a transverse section taken as inclicated by the line 2--2 of Fig. 1:

Fig. 3 is a view similar to Fig. 1 showing the switch in its second operative position;

Fig. 4 is a sectional view of an alternative form of my invention; and,

Fig. 5 is a fragmentary sectional view illustrating a modification of the construction shown in Fig. 4.

The preferred embodiment of the invention shown in Figs. 13 includes a cylindrical switch envelope enclosing a mercury fill H and a displacement means generally designated l2, the displacement means being in the form of an armature for electromagnetic switch actuation. In the normal or de-energized state of the switch, the displacement means I2 is free to float with its lower end partially submerged in the mercury fill as shown in Fig. l. Energization of an exterior electromagnetic means generally designated l3 draws the displacement means 12 down- 4 ward to the position shown in Fig. 2 thereby raising the level of the mercury fill.

The electromagnetic means l3 may comprise a coil l5 having an iron circuit l5 and providing an axial opening to receive and retain the switch envelope Hi. In the particular construction shown in the drawing, the iron circuit includes two circular pole pieces defining an air gap l8 and the switch envelope H) is mounted in proper position relative to the air gap b a rubber collar l9 tightly embracing an insulating sleeve 20. The insulating sleeve 20, which may be Bakelite, completely surrounds the switch envelope I0 and is suitably bonded thereto. Preferably a bracket 24 is adjustabl mounted on the iron circuit It by a machine screw 29 to engage the rubber collar I!) as shown.

The switch envelope |0 comprises a cylindrical wall 2|. an upper cap 22, and a lower cap 23. The cylindrical wall 2| is polished on its inner surface to minimize frictional resistance to movement of the displacement means I2 and is counterbored at its opposite ends to provide shoulders 25 and cylindrical flanges 26 to seat the two caps 22 and 23. The cylindrical wall 2| of the enelope should be made of metal that is nonmagnetic and also resistant to wetting by mercury. A suitable metal, for example, is a stainless steel known as 18-8 Steel having besides iron, approximately 18% chromium and 8% nickel.

The upper cap 22 ma be a metal disc or a pressed sheet metal member with an upturned flange 21 to nest inside of the cylindrical flange 26 against the shoulder 22. The joint between the cap 22 and the cylindrical wall 2| of the envelope may be sealed in any suitable manner, for example, b welding as indicated at 2B. The cap 22 has a central flanged aperture 30 to embrace a body of insulating material 3| having the function of holding a switch electrode 32. The electrode 32 extends into the interior of the switch envelope [0 and at its outer end is adapted for connection with a wire 33 of the circuit to be controlled by the switch.

It is requisite that the materials of the cap 22, the insulating body 3| and the electrode 32 all have substantially the same coeificient of thermal expansion to maintain effective sealing relationships under varying temperature conditions. In my preferred embodiment of the invention, the electrode 32 is made of mblybdenum, the insulating body 3| is a hard glass such as Corning G705AJ fused both to the electrode and to the cap, and the cap 22 is fabricated from a metal known to the trade as Kovar, an alloy manufactured by Stupakoli Laboratories, Pittsburgh, Pennsylvania.

The second wire 35 of the circuit to be controlled by the switch may be soldered to the cap 22 as indicated in the drawing or may be attached to any other part of the switch envelope Ill.

The lower cap 23 likewise is formed with a cylindrical flange 36 to nest against the shoulder 25 at the lower end of the cylindrical wall 2|, the cap being sealed by welding 31. The cap 23, which may be machined from cold rolled steel, is formed preferably with an inner central boss 38 and has an axial bore 40 sealed by a headed pin or plug 4|. In the course of manufacture, air may be evacuated from the welded envelope through the bore 40 and then a suitable inert gas or mixture of inert gases may be introduced through the same bore to fill the envelope under pressure. The gas or gases may be selected from t'erio'r of the switch envelope agro'up comprising hydrogen, nitrogen, argon, and helium hydride. After the gas has been introduced to the desired pressure, the head of the pin or plug 4| is'welded to seal the bore 4 0.

Mounted on the lower cap -23 within the inis a conductor '42 which may be regarded as a second electrode since it cooperates with the first electrode 32 for switch action in a central 'make a'nd break zone. In accord with the basic concept of my invention, the conductor 42 is designed to provide a path of relatively low resistance and relatively high currenman-yiag capacity from the switch envelope to themake and-break zone. This purpose is accomplished by using conducting material'of sufliciently low resistivity and *sufiiciehtly large in cross section. A chromium-plated copper conductor may be preferable because the resistivity of copper F is substantially lower than the resistivity o": the iron or steel of'the displacement means lit. Ach'romiuni-plat'ed iron or steel conductor of largercross section would be equivalent. The conductor may be fabricated from 18 8 steel since it is desirable that the conductor be nonmagnetic and resist wetting by mercury. Cold rolled steel is also satisfactory.

conductor or electrode 42 may be mounted on the lower cap 23 in any suitable manner or may be fabricated in one piece with the cap. In the depicted construction, the conductor is a round rod machined to provide a circular flange 43 at its lower end to snugly and rigidly embrace the cap boss 38. A'press fit may be relied upon to unite theconcluctorwith the cap or the conductor may be welded to the capas indicated at 45; The lower end of the conductor AZ'has a short axial bore 46 and a diametrical bore '4! for communication with the previously mentioned axial bore 40 of the cap to complete the passage forevacuati'ng the envelope and charging the envelope'with inert gas.

In the preferred embodiment of the invention shown in Fig. 1, the conductor or electrode 42 supports a cup for retaining a pool of meronly 50 immersing the end of the upper electrode 32. While the cup may be constructed in any suitable manner and may be mounted in any suitable manner, a feature *of the present form of my invention is the concept'of employing an imperforate cup bo'dy in'mechanical engagement with the conductor'fl, instead of employing a cup body that has an aperture for mounting the cup requiring a sealing agent to prevent the formation of a short-circuiting mercury path through its aperture. Thus, as shown in'the drawing, the cup '48 may he a one piece ceramic body having a stein orprojection 5 I fitting snugly and rigidly into a socket or recess 52 in the i'ipper end of the conductor '42; To prevent retraction of the cup stem, I may insert a cotter pin or rivet 53 diametrically through the cup stem and through the surrounding wall of the socket.

If desired, a bonding substance may be employed instead of, the cotter pin. For example, the

v stem of the cup may be clipped in molten glass before being inserted into the electrode socket, the molten glass cooling to form a strong mechanical bond. As so employed, the bonding substance need not perform the function of a seal and may, in fact, be quite inenicient as a liquid seal. It is apparent that by virtue of the described construction, I am free to employ any desirable ceramic material for the cup without re gard to the coefiicient of thermal expansion of 6 the ceramic mater-iahth'e fusi-bility oithe 1na 'teri'al, or the fineness of it's grain structure.

The displacement means or armature I2 may comprise a cylinder 55 of suitable ferromagnetic material such as iron and may be formed with radial projections or be otherwise designed for guiding cooperation with the surrounding switch envelope. In the present construction, for "ex ample, the cylinder 55 is formed with integral radial flanges 56 at its top and bottom, each of the "flanges being cut away "as shown in Fig. 2 to provide guiding lugs 51 and intervening mercury-flow passages'58. I

Preferably, I employ coil springs 69 of nonmagnetic material at the opposite ends of the ferromagnetic cylinder 55 to serve as buffer means. If either of the caps 22 and 23 is of ferromagnetic material, such coil springs are especially desirable to prevent the displacement means from clinging to the cap magnetically. The term displacement means is to be wider stood either as applying to the ferromagnetic cylinder 55 alone or as covering the cylinder to-- gether with its coil springs 60.

In somepractices of my invention, such as the practices illustrated in the present disclosure, I pl'acea wall ofinsulatin'g material such as glass or-some-suitable ceramic between the displacement means 55 and the cup 48 for the purpose of lengthening the minimum possible path of mercury from the displacement means to the pool 5!! in the cup. Any current flowing between the displacement means and the cup must pass around the ends of such an insulating wall. Obviously the insulating wall may be'fixedly mounted in the switch envelope in any suitable manner ormay be "carried by the ferromagnetic cylinder 55. In my present construction, the insulating Wallis in the form of a cylinder or tube Bl mounted 'inside'the ferromagnetic cylinder in the manner of a liner. Preferably the insulating cylinder 6| ei'ttends beyond the lower end of the through.

ferromagnetic cylinder diameter of both 55 but terminates short of the unrestrained extent of the lower coil spring 60.

The employment of an insulating wall such as the cylinder 6| is' highly desirable because it enables me to design the mercury switch compact- 1y, especially in cross section. It is to be understood, however, that the insulating wall may be omitted in some practices of my invention since the conductance differential required to confine current flow substantially to the conductor or electrode 42 may be achieved by increasing the switch envelope and the displacement "means to provide relatively great radialspa'cing between the cup 48 and the surrounding material of the ferromagnetic cylinder 55.

In thepre'ferred practice or my invention anmeans with a suitable high resistance material selected from materials that will not react with the mercury. For example, I may coat at least the peripheral surface "of the displacement means with magnetic oxide of iron Fe3o4 as stated in my Patent 2,121,804 issued June 28 1938. When The operation of readily understood tion. In Fig. 1, the

the described switch may be from the foregoin descripcoil I5 is tie-energized, displacement means I2 is at its upper position by virtueot its bouyancy with respect ,to the mercury fill, and the mercury fill is at its minimum liquid level 82 substantially below the rim of the cup 48. No current can flow through the switch because the major mercury body is separated from the mercury pool 50. No short circuit through the cup is possible because the cup is of unitary imperforate construction. Energization of the electromagnet draws the displacement means I2 to its lower position shown in Fig. 3 thereby dis placing the mercury to the maximum liquid level 63, the maximum liquid level being above the rim of the cup 48 to cause the major mercury body to merge with the pool 50 to .form a conducting bridge between the upper electrode 32 and the lower electrode 42.

It is to be noted that in the closed position of the switch shown in Fig. 3, current flow between the two electrodes passes through mercury for only a short distance since the conductor 42 extends upwardly close to the bottom of the mer-- cury cup, and extends laterally substantially to a vertical plane at the side of the cup. On the other hand. any current flowing between the displacement means I! and the upper electrode 32 must pass under the lower edge of the insulating cylinder Bl to traverse an excessive dis tance through the mercury. Since the resistivity of the mercury is so much greater than the re-- sistivity of any of the structural metals, the disparity in length of the available mercury paths may alone account for the required favorable con" ductance differential. ,The conductance differential is, however, augmented by making the conductor 42 relatively massive.

The second embodiment of my invention shown in Fig. 4 is largely similar to the first described form, as indicated by the use of corresponding numerals to designate corresponding parts. In this second form of the invention I employ a relatively extensive upper electrode Ill and mount a mercury cup II On the lower end of the electrode. Here again, the ceramic body of the cup is unitary and imperforate to avoid any possibility of a short circuit through the cup wall.. For mounting the cup II on the electrode 10, I rely upon a mechanical relationship, for example, forming the lower end of the electrode with screw threads to engage complementary threads in an axial recess 12 in the bottom of the cup.

A lower conductor or electrode 16, shown in Fig. 4 does not have the function of supporting the cup II, and therefore need not make actual contact with the cup. It does, however, have the function of creating the required conductance differential and therefore is relatively large in cross section and extends upward a. sufficient distance to materially shorten the mercury path between the two electrodes. Preferably the lower conductor extends laterally, as shown, to substantially a vertical plane at the side of the cup so that current may flow in substantially a straight line from the lower electrode along the side of the cup to the mercury p001.

Instead of relying on a screw thread engagement between the upper electrode and the cup H or like mechanical relationship, I may use a bonding substance. Thus, as indicated in Fig. 5, the screw threads may be omitted from both the electrode and the cup recess and an intervening layer 11 of glass or other suitable material may be relied upon to hold the cup rigidly on the lower end of the electrode.

Either the switch shown in Figs..1-3 or the switch shown in Fig. 4 may be designed as a front contact switch or a back contact switch, that is to say, may be normally open or normally closed when the electromagnet coil is de-energized. It will be notedthat the specific form shown in Figs. 1-3 is a front contact switch while the specific form shown in Fig. 4 is a back contact switch.

- Fig. 4 shows the displacement means 12 floating on the mercury fill with the liquid level of the fill sufficiently high to immerse the cup II. Upon energization of the coil l5, the displacement means is lifted from its normal floating position sufficiently to cause the liquid level of the mercury fill to drop substantially below the lip of the cup II to open the circuit.

My description, in specific detail, of preferred forms of the mercury switch will suggest to those skilled in the art various changes and substitutions under my basic concept and I reserve the rights to all such departures from my description that properly lie within the scope of my appended claims.

I claim:

i. In a mercury switch. a metal switch envelope for connection with one side ol a circuit, a mercury fill in the envelope, 3. metal displacement means of hollow configuration in said envelope, means to shift said displacement means thereby to vary the liquid level of the mercury fill, an electrode for the other side of the circuit having a portion exposed for mercury contact between the minimum and maximum levels of the merfill, and a conductor of lower specific reuse than mercury extending from the wall oi .aid envelope to a point sufficiently near said exposed portion of said electrode to provide a mercury path to the exposed portion substantially shorter than mercury paths to said exposed portion from any points on said displacement means whereby said conductor cooperates with the mercury fi l to provide a path from said envelope to sa d exposed portion of substantially greater conductance than any path through said displacement and thereby prevents damaging current flow between the envelope and the displace- 1r out means under relatively high switch loading.

2. In a mercury switch, a metal switch envelope for connection with one side of a circuit, a mercury fill, a metal displacement means of hollow configuration in said envelope, said displacement means being shaped and dimensioned for guidance by the envelope, means to shift said displacement means thereby to vary the liquid level of the mercury fill, a cup fixedly supported within the interior of said displacement means to hold a pool of mercury between the minimum and maximum liquid levels of the mercury fill, an electrode for the other side of the circuit contactin said pool, and a. conductor of lower specific resistance than mercury extending from the wall of said envelope to a point sufficiently near said cup to provide a. mercury path to the interior of said cup substantially shorter than mercury paths to the interior of the cup from any points on said displacement means whereby said conductor cooperates with the mercury fill to provide a path from said envelope to said pool oi substantially greater conductance than any path through said displacement means and thereby prevents damaging current flow between the envelope and the displacement means under relatively high amperage switch loading.

3. In a mercury switch, a metal switch envelope for connection with one side of a circuit, a mercury fill, a metal displacement means of hollow configuration in said envelope, said displacement means being shaped and dimensioned for uidance by the envelope, means to shift said displacement means thereby to vary the liquid level of the mercury fill, a cup fixedly supported within the interior of said displacement means to hold a pool of mercury between the minimum and maximum liquid levels of the mercury fill, an electrode for the other side of the circuit contacting said pool, and a conductor of relatively large cross section of metal having substantially lower specific resistance than mercury and substantially lower specific resistance than the material of said displacement means, said conductor extending from the wall of said envelope to a point sufficiently near said cup to provide a mercury path to the interior of said cup substantially shorter than mercury paths to the interior of the cup from any points on said displacement means whereby said conductor cooperates with the men cury fill to provide a path from said envelope to said pool of substantially greater conductance than any path through said displacement means and thereby prevents damaging current flow between the envelope and the displacement means under relatively high amperageswitch loading.

4. A mercury switch as set forth in claim 3 in which said conductor is formed with a passage for communication with the exterior of said envelope in the course of manufacture and in which means is provided to seal said passage.

5. In a mercury switch, a metal switch envelope for connection with one side of a circuit, a mercury fill, a metal displacement means movable in such envelope, means to shift said displacement means thereby to vary the liquid level of the mercury fill, a cup fixedly supported in said envelope to hold a pool of mercury above the minimum liquid level of said fill, an electrode for the other side of the circuit contacting said pool, a nonconducting wall between said displacement means and said cup to lengthen the path of conduction through the mercury fill from the displacement means to said pool, and a conductor of substantially less specific resistance than mercury connected with said envelope, said conductor extending toward said cup to lessen the path. of conduction through the mercury from the envelope to the pool whereby the conductance from the envelope to the pool through said conductor is substantially greater than the conductance from the envelope to the pool through said displacement, the conductance differential being suificient to preclude damaging current fiow between the envelope and the displacement means under relatively high amperage switch loading.

6. In a mercury switch, a metal switch envelope fOr connection with on side of a circuit, a mercury fill, a metal displacement means movable to vary the liquid level of said mercury fill, means to actuate said displacement means, metal bufier means yieldingly extending from the submerged end of said displacement means, a cup fixedly supported in said envelope to hold a pool of mercury between the minimum and maximum liquid levels of the mercury fill, an electrode for the other side of the circuit contacting said pool, a wall of insulating material between said displacement means and said cup to lengthen the path of conduction through the mercury from the displacement means to said pool, said wall being carried by said displacement means and terminating short of the unrestrained extent of said buffer means, and a conductor of substantially less resistance than mercury extending from said envelope toward said cup to lessen the path of conductance from the envelope to the pool through the mercury whereby the conductance from the envelope to the pool thr gh sa conductor is sufficiently greater than the conductance from the envelope to the pool through said displacement means to preclude any damaging current fiow between the envelope and the displacement means under relatively high amperage switch loading.

7. In a mercury switch, a metal switch envelope for connection with one side of a circuit, a mercury fill, a metal displacement means of hollow configuration within said envelope having peripheral portions for guiding cooperation with the envelope, means to shift said displacement means thereby to vary the level of the mercury fill, a cup to hold a pool of mercury above the minimum liquid level of the fill, an electrode for the other side of the circuit contacting said pool, and a support means rigidly carried by the envelope for supporting said cup, said support means being of material of substantially lower specific resistance than mercury and extending to a point providing a mercury path to the interior of said cup substantially shorter than mercury paths to the interior of the cup from any points on said displacement means whereby said support means cooperates with the mercury fill to provide a path from said envelope to said pool of substantially greater conductance than any path to said pool through the displacement means and thereby prevents damaging current flow between the envelope and the displacement means under relatively high amperage switch loading.

8. In a mercury switch, a metal switch envelope for connection with one side of a circuit, a mercury fill, a metal displacement means movable in the envelope to vary the liquid level of the mercury fill, means to actuate said displacement means, a non-metallic cup to hold a pool of mercury between the minimum and maximum liquid levels of said fill, a first electrode for the other side of the circuit contacting said pool, and a second electrode extending from a wall of said envelope to said cup to support the cup, said second electrode in at least one transverse dimension extending substantially to the side of the cup thereby providing a mercury path from the second electrode along the side of the cup to said pool, substantially shorter than the shortest mercury path from the displacement means to the pool to prevent damaging current flow between the switch envelope and the displacement means at relatively high amperage switch loading.

CARL H. LARSON. 

